1. Field of the Invention
The present invention relates to a method and apparatus for forming silver halide emulsion particles and method for forming fine particles, and more particularly, to a technology of forming silver halide emulsion particles using a static mixing apparatus.
2. Description of the Related Art
Forming silver halide emulsion particles used for silver halide photosensitive material involves two major steps. One is a nucleus formation step of forming seed particles of silver halide emulsion particles and the other is a particle growth step of allowing seed particles to grow to particles in sizes suitable for photosensitive material.
First, in the nucleus formation step, preparing uniformly flat-shaped particles requires an increase of the probability of occurrence of double twin crystals with a uniform size distribution of particles in a stage of seed particles which become the seeds of flat-shaped particles. In order to allow such flat-shaped particles to grow, it is effective to add seed particles for growth formed in the nucleus formation step to a system in which growth host particles exist to thereby regulate the growth direction and develop Ostwald maturation. Such seed particles are required to have microsizes and an excellent mono-dispersion characteristic.
In forming such seed particles, if an aqueous solution of silver salt (hereafter will be explained in an example of “aqueous solution of silver nitrate”) and aqueous solution of haloid salt are mixed in quite a low concentration state and allowed to react with each other by a mixing reactor, it is possible to form seed particles of desired twin crystals except excessively weak stirring or mixing conditions, but use under low concentration conditions is not industrially profitable. Thus, forming seed particles at an industrially profitable concentration level or letting seed particles grow requires reaction under high concentration conditions.
Stably forming minute silver halide emulsion particles in the nucleus formation step or particle growth step requires considerations of the apparatus to prevent nucleus formation and growth of particles from simultaneously occurring and it is desirable to use a small volume static mixing apparatus that will not cause backflow as the mixing apparatus for that purpose. Here, the static mixing apparatus refers to a mixing apparatus that has no stirring device such as a stirrer at the mixing site.
Methods for forming silver halide emulsion particles using such a static mixing apparatus are disclosed in Japanese Patent Application Publication Nos. 4-292416, 11-217217, 2000-187293, etc., and these methods allow an aqueous solution of silver nitrate and aqueous solution of haloid salt to mix and react with each other instantaneously by allowing two jet flows of the aqueous solutions of a high Re (Reynolds number) to collide with each other at an intersection of very narrow pipes such as T-shaped pipes or Y-shaped pipes and discharge the liquid resulting from mixing reaction in a short time.
In the case of a conventional static mixing apparatus, the jet flow speed should be increased to increase the mixing efficiency by allowing both liquids, which are high-speed turbulent flows, to collide with each other. However, when the jet flow speed is increased, frictional heat is generated by friction between both liquids. Since reaction for forming silver halide emulsion particles is heating reaction, if frictional heat is added to the heating reaction, growth through Ostwald maturation advances in the seed particles formed by reaction between an aqueous solution of silver nitrate and aqueous solution of haloid salt, causing the defect that it is difficult to form silver halide emulsion particles which are fine particles with a good mono-dispersion characteristic.
Furthermore, a high-speed turbulent jet flow is liable to cavitation, and bubbles by the cavitation gather together easily forming a gas-liquid interface in the static mixing apparatus, which produces unevenness in mixing and reaction, resulting in the defect that it is difficult to form silver halide emulsion particles which are fine particles with an excellent mono-dispersion characteristic.